Detection tag

ABSTRACT

PCT No. PCT/NL93/00239 Sec. 371 Date Jul. 31, 1995 Sec. 102(e) Date Jul. 31, 1995 PCT Filed Nov. 12, 1993 PCT Pub. No. WO94/12957 PCT Pub. Date Jun. 9, 1994A detection tag for a resonance detection system. A resonance circuit is supported on an electrically insulative support, and includes and inductive track in a predefined pattern and a capacitive element with capacitor electrodes separated from each other by the support and connected to the respective ends of the inductive track. An electroconductive island is provided on the first side of the support with one of the capacitor electrodes, and is separated from that capacitor electrode by a discharge gap distance, and is also separated from the capacitor electrode on the other side of the support.

BACKGROUND OF THE INVENTION

The invention relates to a detection tag for a resonance detectionsystem, comprising a support consisting of electrically insulatingmaterial and a resonant circuit supported thereby, which has aninductive element formed by a conductor track disposed on the support ina predefined patter, and a capacitive element formed by at least twocapacitor electrodes which are kept apart by the support and areconstructed as electroconductive electrode regions, the ends of thetrack being connected to the one and to the other capacitor electrode,respectively.

A detection tag of this type is disclosed by the laid-open EuropeanPatent Application 0 463 233 A2.

The support of this known tag is provided on one side with a conductortrack according to a spiral and rectangular pattern and on both sides isprovided with electroconductive regions which form capacitor electrodesor plates of 4 capacitors. The capacitive element of the support isformed by a connection in parallel of 2 branches, in which twocapacitors connected in series are incorporated. Said capacitive elementis connected to the ends of the spiral track, as a result of which aresonant circuit is obtained having a resonant frequency which differsfrom a detection frequency which is used in an anti-theft system. Thedetection tag is activated by a capacitor being short-circuited in orderto tune the resonant circuit to the detection frequency. If theactivated detection tag has to be deactivated, a following capacitor isshort-circuited, so that the resonant frequency of the tag once morediffers from the detection frequency of the anti-theft system.

The capacitors to be short-circuited are provided with an indentation,as a result of which the corresponding plates are situated at a reducedmutual distance locally. The first capacitor is short-circuited byelectromagnetic energy being supplied with a frequency which correspondsto the current resonant frequency of the tag and at a level which issufficiently high to produce a discharge transversely to the support atthe indentation of the capacitor in question. Short-circuiting of thesecond capacitor is effected in corresponding manner.

The known tag has the drawback that as a result of using indentations inthe capacitors the resonant frequencies are not precisely defined, sothat high energy levels or an additional tuning action are necessary.

SUMMARY OF THE INVENTION

The object of the invention is to provide a detection tag of the typementioned in the preamble, which overcomes the abovementioned drawback.

This object is achieved according to the invention by at least oneelectroconductive island region being disposed on the support so as tobe adjacent to, and in the same plane as, one of the capacitorelectrodes, those edges of the island region and the capacitorelectrode, which face one another, being situated at a discharge gapdistance.

This arrangement has the advantage that, for the purpose of predefininga discharge path by means of the discharge gap, neither the qualityfactor nor the resonant frequency of the resonant circuit of thedetection tag are adversely affected. Both variables remain preciselydefined, even after the discharge, and indeed are not subject toscatter.

It should be noted that the European Patent Application 0 458 923 doesdisclose a discharge along the surface of the support, but this is usedfor short-circuiting a capacitor and not for increasing the capacitanceof said capacitor. Moreover, an additional connection through thesupport is required.

Preferred embodiments are specified in the subordinate claims.

DESCRIPTION OF THE DRAWINGS

The invention will be described below in more detail with reference tothe drawings, in which:

FIG. 1 shows an embodiment of the invention with two possible resonantfrequencies;

FIG. 2 illustrates another embodiment of the invention with two possibleresonant frequencies; and

FIG. 3 depicts a further embodiment of the invention with four possibleresonant frequencies;

FIG. 4 shows yet another embodiment of the invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

The detection tags shown in the figures can be used in an electronicdetection system (not shown). It is generally known that a system ofthis type is used in shops to protect the articles present there againsttheft. An electronic protection system of this type is described, forexample, in U.S. Pat. Nos. 4,692,744 and 4,831,363.

The known anti-theft system comprises a transmitter for emitting to, andgenerating electromagnetic fields in, a detection zone. Preferably aradio-frequency electromagnetic field having a predefined frequency,hereinafter called detection frequency. A frequency of 8.2 MHz is asuitable frequency, although other frequencies may also be used.

The electronic protection system further comprises a receiver fordetecting the presence of a detection tag in the detection zone, byreason of this tag having a resonant frequency which is virtuallyidentical to the detection frequency of the electromagnetic field. Thistag is brought into resonance by the electromagnetic field, which isdetected by the receiver.

The European Patent Application 0 463 233 describes an activableanti-theft tag which can be attached to an article to be protected. Saidprotection tag consists of a support made of electrically insulatingmaterial which supports a resonant circuit. The inductive portion of theresonant circuit is formed mainly by a conductor track disposed on thesupport in a spiral patter. The capacitive portion, supported by thesupport, of the resonant circuit is formed by a capacitor which in itsinitial state with the spirally wound coil has a first resonantfrequency which differs from the detection frequency of the protectionsystem. Said known detection tag is provided with means for altering thecapacitance of the capacitor, in such a way that in the activated statethe resonant frequency of the resonant circuit is equal to the detectionfrequency, while in the deactivated state the resonant frequency isagain altered to a third frequency value. As long as the product has notbeen paid for at the till, the detection tag has a resonant frequencywhich is equal to the detection frequency of the security system, whileafter payment the tag is set to a deactivated state, in which theresonant frequency of the resonant circuit once more differs from thedetection frequency of the security system, so that no theft detectionwill take place when the article with the detection tag is taken throughthe detection zone.

The known alteration of the capacitance value of the capacitive elementof the resonant circuit is effected according to the European PatentApplication in a known manner by means of a discharge transverselythrough the support, as a result of which a portion of the capacitiveelement is reduced in size each time.

FIG. 1 shows a detection tag according to the invention, in which thedischarge takes place along the surface of the support.

This detection tag consists of a support 1, to which a conductor track 4in the form of a spiral is applied. Said spiral-shaped track forms acoil having a predefined self-inductance. At one end, the track 4 isconnected to a region 7 which is disposed on the same side of thesupport 1 and consists of electroconductive material. This region 7forms one capacitor electrode of a capacitor, whose other capacitorelectrode is formed by the region 5 which is disposed on the other sideof the support 1 and consists of electroconductive material. This region5 is connected by means of a track 9 to a connection region 2, likewiseconsisting of electroconductive material, which is connected through thesupport 1 to the connection region 3 of the track 4. Thus a resonantcircuit is formed, in which the number of windings of the conductortrack 4 and the area of the regions 5 and 7 are dimensioned in such away that the resonant frequency of the resonant circuit is equal to thedetection frequency of the electronic detection system to be used.

Adjacent to the region 7, there is disposed in the form of an island aregion 6, likewise consisting of electroconductive material, on the sameside of the support as the region 7.

Those edges of the regions 6 and 7, in which face one another, are atsuch a distance that a discharge is produced between the edges if thetag is subjected to an electromagnetic field whose frequency is equal tothe resonant frequency or detection frequency which is determined by theself-inductance formed by the track 4 and the capacitance formed by thecapacitor plates 5 and 7, and if the energy level of the electromagneticfield is sufficiently high to achieve this. This discharge gives rise toan electrical connection between the regions 7 and 6, so that the areaof the capacitor electrode corresponding to the region 7 is increased bythe area of the region 6. As a result, the detection tag is set to aresonant frequency which is reduced with respect to the detectionfrequency, so that the detection system will not react if this tag ismoved into the detection zone.

As shown in FIG. 1, the electrode region 5 overlaps the island region 6.Depending on the area of the region 6 and the degree to which the region5 overlaps the region 6, an enlargement of the capacitor and,consequently, a corresponding reduction of the resonant frequency isachieved.

The support 1 may, for example, consist of a flexible plastic filmhaving a thickness of 20 μm, such as, for example, polyethylene. Thisflexible support has the track 4 and the conductive regions 2, 3, 5, 6and 7 disposed thereon by means of, for example, a deposition or etchingprocess. The conductive material may consist of aluminium with athickness of, for example, from 15 to 50 μm.

According to FIG. 1, a well-defined discharge gap 8 is formed as aresult of the distance between the edges facing each other of theregions 6 and 7 being reduced locally to, for example, less than 5 μm.Experiments have shown that a voltage of from 80 to 90 volts between thegap edges is sufficient to produce a discharge.

The detection tag according to the invention has the advantage that thequality factor of the resonant circuit is not affected by the additionof the discharge gap, and this factor is accurately defined even afterthe discharge process. Moreover, the resonant frequencies can be setrapidly and easily during fabrication, for example by means of a laserbeam, while remaining well-defined, since the discharge will not affectthem. This provides for more accurate detection than in the knowndetection tags.

Owing to the fact that the resonant frequencies can be defined moreaccurately and the quality factor and the resonant frequency remainwell-defined at all times, the detection tag can easily be extended to aplurality of resonant frequencies. A preferred embodiment is shown inFIG. 3.

The embodiment shown in FIG. 3 provides the possibility of four resonantfrequencies. For the sake of clarity, the inductive component of theresonant circuit is not shown.

The capacitor supported by the support 1 consists of the capacitorelectrode 7 which by means of the connection 4 is connected to one endof the inductive component (not shown). Disposed on the other side ofthe support 1, there is the other capacitor electrode 5 which by meansof the connection 9 is connected to the other end of the inductivecomponent (not shown) of the resonant circuit of the tag. In addition tothe island region 6 there is disposed, adjacent thereto, another islandregion 10 on the same side of the support 1. On the other side of thesupport 1, a further island region 11 is applied. Between the regions 7and 6, 6 and 10 and 5 and 10, respectively, discharge gaps 8 arepresent.

The resonant frequency, which is determined by the capacitance betweenthe regions 5 and 7 on the one hand, and the inductive component (notshown) is, for example 8.2 MHz. If the detection tag is subjected to anelectromagnetic field having a frequency of 8.2 MHz and a sufficientlyhigh energy level, a discharge is produced between the discharge gap 8between the regions 6 and 7, as a result of which the resonant frequencyof the resonant circuit of the detection tag is lowered to, for example,6.2 MHz. Said resonant frequency obviously depends on the dimensions ofthe regions 6 and 7 and the self-inductance of the inductive componentof the detection tag. In a similar manner, a discharge can be broughtabout successively between the remaining discharge gaps 8, as a resultof which resonant frequencies of, for example, 5 and 4 MHz,respectively, can be achieved.

The number of resonant frequencies can be increased to a virtuallyunlimited extent. For example, the first resonant frequency can be addedto the initial rest state of the detection tag, while a second frequencycan be added to the activated state of the detection tag. Said secondresonant frequency is then used for detecting a theft. The otherresonant frequencies can then be used for coding miscellaneousinformation such as, for example, the number of articles bought, andother information. It is evident that the detection system must beextended in accordance with the number of possible resonant frequenciesof the detection tag.

In order to code an item of information on the detection tag, anelectromagnetic field is preferably used having a frequency swing whichis set to obtain a preselected resonant frequency.

In a preferred embodiment of the detection tag, the conductor track isdisposed spirally around the area occupied by the capacitor regions andconductive regions. The advantage of this is that no additionalconnections are required between the regions on the one hand and thespiral track.

FIG. 2 shows another embodiment of the invention, in which it ispossible, by means of an electromagnetic island region, to increase theinitial resonant frequency. This detection tag consists of a support 1,on which a spiral track 4 is disposed which, by means of the connectionregions 3 and 2, the through-connection effected between said tworegions through the support 1, and the connection 9, is connected to thecapacitor electrode 5 on the other side of the support 1. The othercapacitor electrode 7 is connected to the other end of the spiral track4. This configuration defines a first resonant frequency. A second,higher resonant frequency is obtained by an island region 13 in theshape of a spiral which is disposed within the spiral 4. The spiral 13is connected to the spiral 4 by means of the connection 14, while theother end of the spiral 13 is disposed at a small distance 12 from theopposite end of the spiral 4. The distance 12 defines a discharge gap.If the detection tag is subjected to an electromagnetic field having afrequency which is equal to the initial resonant frequency of the tag, adischarge between the discharge gap 12 is brought about, as a result ofwhich the self-inductance of the resonant circuit is increased, and asecond, high resonant frequency is obtained. A lowering of the resonantfrequency can be achieved once more by a discharge between the dischargegap 8 which is situated between the regions 6 and 7.

The track 4 may also run within track 13, but it is also possible to addmore tracks with discharge gaps which correspond to the track 13.

In the embodiment shown in FIG. 4, the discharge gap 8, 12 is bridged bya resistor in the form of a resistor track 15. If no discharge has takenplace so far and the resistor, for example, bridges a gap between twoadjacent capacitor electrode regions, the circuit consists of a parallelconnection of an inductor and a parallel subconnection of a firstcapacitor and a series-connection of a second capacitor and a resistor.As a result, the resonant frequency is shifted somewhat compared to aconfiguration without a resistor, while the quality factor of thecircuit is somewhat reduced, depending on the resistance which may, forexample be 1 kΩ or higher. After a discharge across the gap has beencarried out, the resistor is short-circuited, while the quality factorof the circuit has increased again.

The same effects occur if the bridging resistor is connected in parallelwith the inductors (see FIG 2: 12). It was therefore found that thedischarge causes both a well-defined frequency change and a qualitychange of the circuit. As a result, an amplitude and a decay behaviourare observed which depend on whether or not a discharge has been carriedout. This embodiment has the advantage that detection can take placebased on amplitude, frequency, phase and/or decay time.

Moreover, the invention has the advantage that prior to or following adischarge, it is possible to test whether the circuit has been damaged.

In general, the invention has the advantage that after each discharge aresidual resonance remains present at all times, so that it is possibleto detect whether or not the circuit has been damaged. Application ofthe invention further makes available a detection tag which can bereused after activation by discharge. After all, the through-connectionbetween adjacent regions, caused by the discharge, can be removed againby supplying energy at a high level. The original state with a dischargegap is thus obtained.

It is evident that the detection tag according to the invention issuitable not only for detecting theft, but also for detecting otherinformation.

We claim:
 1. A detection tag for a resonance detection system,comprising a support consisting of electrically insulating material anda resonant circuit supported thereby, which has an inductive elementformed by a conductor track disposed on the support in a predefinedpattern, and a capacitive element formed by at least two capacitorelectrodes which are kept apart by the support and are constructed aselectroconductive electrode regions, the ends of the track beingconnected to the one and to the other capacitor electrode, respectively,wherein at least one island region (13) is disposed on a first side ofthe support (1) so as to be adjacent to, and in the same plane as, theconductor track (4) and so as to be separated from a second side of thesupport, in that one end of the island region is connected to one end ofthe track, and in that there is present, between the edges thereoffacing each other, a discharge gap (12).
 2. The detection tag accordingto claim 1, wherein the conductor track (4) and the island region (13)have a spiral shape and run between one another.
 3. The detection tagaccording to claim 1, wherein at least one discharge gap is bridged by aresistor.
 4. The detection tag according to claim 1, wherein the tag issubjected to an electromagnetic field having a frequency swing which isset so as to obtain at least one preselected resonant frequency on thesupport.
 5. The detection tag according to claim 1, wherein at least onedischarge gap is bridged by a resistor.
 6. The detection tag accordingto claim 1, wherein the tag is subjected to an electromagnetic fieldhaving a frequency swing which is set so as to obtain at least onepreselected resonant frequency on the support.
 7. A detection tag for aresonance detection system, comprising:an electrically insulativesupport; a resonance circuit supported by said support, said resonancecircuit comprising an inductive track in a predefined pattern, acapacitive element with capacitor electrodes separated from each otherby said support and connected to respective ends of said inductivetrack; and first electroconductive island on a first side of saidsupport with a first one of said capacitor electrodes, said first islandbeing separated from said first capacitor electrode by a discharge gapdistance and separated from a second said capacitor electrode on asecond side of said support by said support.
 8. The tag of claim 7,wherein said first island overlaps at least a portion of said secondcapacitor electrode, whereby an increase in the overlap increase thecapacitance of said resonance circuit.
 9. The tag of claim 7, furthercomprising at least a second said electroconductive island on said firstside of said support and which is separated from said first island by afurther discharge gap distance.
 10. The tag of claim 9, furthercomprising at least a third said electroconductive island on said secondside of said support and which is separated from said second capacitorelectrode by a discharge gap distance and separated from said firstisland by said support.
 11. The tag of claim 7, further comprising aconstriction between said first island and said first capacitorelectrode, said constriction defining the discharge gap distance. 12.The tag of claim 7, further comprising at least a second saidelectroconductive island on said second side of said support which isseparated from said second capacitor electrode by a discharge gapdistance and separated from said first island by said support.
 13. Thetag of claim 7, wherein a portion of said track on said first sidespirals around said first capacitor electrode and said first island. 14.The tag of claim 13, wherein a portion of said track external to saidspiral portion extends through said support to said second side.